The present invention is directed to nuclear reactor control instrumentation systems. More particularly, the invention is directed to a neutron detection system, which is operable to detect thermal neutron fluxes over a wide range of flux levels. In the past, it has been found necessary to use a variety of different neutron detectors to cover the entire range of neutron flux levels for a typical light water reactor. For high neutron flux levels, direct current operation mode ionization chamber detectors have been utilized, while for low neutron flux levels, a proportional counter detector operated in the pulse mode has been utilized. The ionization chamber can be gamma compensated to improve the accuracy of the signal level.
The fission chamber is an ionization chamber type neutron detector which is gas filled, and which is used to detect neutron flux by the reaction of penetrating thermal neutrons with an amount of fissionable material which is included within the device. Fissionable material may typically be uranium 235. The fission fragments released by the reaction with the thermal neutrons ionize the fill gas which will result in detection of a pulse signal in low flux levels whereas an average DC current signal is measured at high flux levels. The direct current mode of operation of such fission chamber devices is limited at the maximum flux level, typically about 10.sup.10 neutrons per square centimeter per second, by the saturation characteristics of the detector. The minimum detectable neutron flux level in the direct current mode of operation is typically about 10.sup.6 neutrons per square centimeter per second, and is determined by the direct current alpha signal background current in the chamber. The pulse counting mode of operation is limited at the upper level of about 10.sup.5 counts per second by the resolution of individual pulses in the chamber, and at the lower level of one count per second by background radiation and/or counting statistics. These modes of operation do not overlap and two separate detector chambers are required to form a system which covers the entire operational neutron flux range for a typical reactor control system.
It is desirable to be able to utilize a single detector which can effectively cover the entire range of neutron flux in an operational reactor to minimize the number of components and the space requirements.
A variety of gamma compensated ionization chamber designs are well known in the art such as U.S. Pat. No. 2,852,694. A variety of alpha particle compensation means for neutron detectors are also well known in the art.